Acoustic array wellbore logging tools are known in the art for determining acoustic properties of earth formations penetrated by a wellbore. The array acoustic wellbore logging tools known in the art typically include an energy emitting transducer, called a transmitter, and a plurality of receiving transducers, called receivers, axially spaced apart from the transmitter along an elongated tool mandrel. The tool is typically lowered into the wellbore at one end of an armored electrical cable. The transmitter periodically energizes the wellbore with pulses of acoustic energy. The acoustic energy travels through a fluid filling the wellbore and interacts with the interface between the fluid and the wall of the wellbore. Some of the acoustic energy can then travel along the wellbore wall. After traveling along the wall of the wellbore, some of the energy can travel back into the wellbore towards the tool where it can be detected by the receivers. The receivers convert the acoustic energy into electrical signals having amplitudes corresponding to the acoustic energy amplitude.
A commonly used method for analysis of the acoustic signals is the so-called semblance technique. For example, a method which is known in the art for determining acoustic transmission velocity of the formation, the method being called semblance correlation, comprises determining values of time difference between the signals from each of the receivers at which the degree of correspondence between the signals reaches a maximum. The time difference is then used to calculate a formation acoustic velocity since the distances between the receivers are known. See, for example, U.S. Pat. No. 4,594,691 to Kimball et al., U.S. Pat. No. 5,541,890 to Tang, U.S. Pat. No. 6,023,443 to Dubinsky et al., U.S. Pat. No. 6,930,616 to Tang et al.
In a cased hole environment, the presence of the casing often prevents the measurement of the formation compressional wave slowness especially when the casing is poorly bonded. This is due to the fact that when an acoustic source is activated, the casing will ring. The ringing signal propagates with substantially no attenuation and will overwhelm the formation arrival. If, on the other hand, the casing is well bonded to the formation, the ringing of the casing is not an issue. The present invention is a method of determination of formation compressional and shear-wave velocities in a cased borehole.